lib: create 'my' extensions

Inspired by this [1] repo, I want to add some functions to `lib.my`. I
would have liked to make it work without having to use an overlay, but I
did not manage to do it...

[1]: https://github.com/hlissner/dotfiles

lib/default.nix

@@ -0,0 +1,21 @@
+# Extension mechanism shamelessly stolen from [1].
+#
+# [1]: https://github.com/hlissner/dotfiles/blob/master/lib/default.nix
+{ lib, pkgs, inputs }:
+let
+  inherit (lib) makeExtensible attrValues foldr;
+  inherit (modules) mapModules;
+
+  modules = import ./modules.nix {
+    inherit lib;
+    self.attrs = import ./attrs.nix { inherit lib; self = { }; };
+  };
+
+  mylib = makeExtensible (self:
+    with self; mapModules ./.
+      (file: import file { inherit self lib pkgs inputs; })
+  );
+in
+mylib.extend (self: super:
+  foldr (a: b: a // b) { } (attrValues super)
+)